The present invention relates to a thermally insulated composite article or conduit adapted to conduct high temperature gases such as the exhaust gases of an internal combustion engine. The article comprises (1) a core composed of ceramic material and defining at least one flow passage, and (2) a metal jacket cast directly around the core.
Articles of the above-described type are generally known and have been employed, for example, as exhaust gas conduits for internal combustion engines, particularly for motor vehicles. Specific examples of such conduits are disclosed in the U.S. Pat. Nos. 3,568,723 and 3,709,772. The above-noted construction comprising an inner ceramic core and an outer metal jacket cast directly around it advantageously provides for thermal insulation of the exhaust gases flowing through the exhaust conduit, so that the temperature in the exhaust conduit can be kept at a level which offers the most favorable conditions for after-burning those exhaust gas components which were burned only incompletely in the cylinders. With this construction--as opposed to an arrangement in which the metal is surrounded by heat insulation--the metallic conduit jacket is advantageously subjected only to low temperatures, so that consequently also the cylinder head, to which the conduit is directly flanged, is not subjected to any additional thermal stresses caused by heat conduction. Furthermore, since the metallic conduit jacket is manufactured by casting the metal around the ceramic core, a solid bond is obtained between the core and the jacket that is maintained even under the extreme conditions of heat expansion during engine operation.
In order to permit the full-scale commercial use of the above-described article in motor vehicles, it is necessary to find a suitable material for the ceramic core that will satisfy the requirements which this article or conduit is expected to meet. In addition to a chemical resistance to the components contained in the exhaust gases and an adequate strength to withstand the pressure exerted by the cast metal as it solidifies, the material used for the preparation of the core must exhibit superior strength and thermal resistance under conditions of thermal shock since, during the operation of an exhaust gas conduit, the core is subjected to peak temperatures of up to 1400.degree. C. and continuous temperatures of up to 1000.degree. C. In particular, the core material must neither break or tear nor suffer any other damage in the face of the temperature differences of substantial magnitude which occur during casting as well as operation in the field.